Homeostasis

1. Homeostasis and Feedback LoopsEQ: What is homeostasis and how are feedback loops used to achieve it?

2. Homeostasis • Defined as maintenance of a relatively stable internal environment • Does not mean that composition, temperature, and other characteristics are absolutely unchanging • Homeostasis is essential for survival and function of all cells • Each cell contributes to maintenance of a relatively stable internal environment

3. Homeostasis • Homeostasis involves dynamic mechanisms that detect and respond to deviations in physiological variables from their “set point” values by initiating effector responses that restore the variables to the optimal physiological range. • Two systems that maintain homeostasis are: Nervous system & Endocrine system

4. Maintenance of Homeostasis • Nervous system • Controls and coordinates bodily activities that require rapid responses • Detects and initiates reactions to changes in external environment • Endocrine system • Secreting glands of endocrine regulate activities that require duration rather than speed • Controls concentration of nutrients and, by adjusting kidney function, controls internal environment’s volume and electrolyte composition

5. Homeostasis Factors homeostatically regulated include • Concentration of nutrient molecules • Concentration of water, salt, and other electrolytes • Concentration of waste products • Concentration of O2 = 100mmHg and CO2 = 40 mmHg • pH = 7.35 • Blood volume 4-6 L and pressure 120/80 • Temperature = 37o C

6. Control of Homeostasis • Homeostasis is continually being disrupted by • External stimuli • heat, cold, lack of oxygen, pathogens, toxins • Internal stimuli • Body temperature • Blood pressure • Concentration of water, glucose, salts, oxygen, etc. • Physical and psychological distresses • Disruptions can be mild to severe • If homeostasis is not maintained, death may result

7. Feedback Loops: Types • Negative feedback loop • original stimulus reversed • most feedback systems in the body are negative • used for conditions that need frequent adjustment • Positive feedback loop • original stimulus intensified • seen during normal childbirth

8. Negative Feedback Loop • Negative feed back loop consists of: • Receptor - structures that monitor a controlled condition and detect changes • Control center - determines next action • Effector • receives directions from the control center • produces a response that restores the controlled condition

9. Negative Feedback Loop

10. Homeostasis – Negative Feedback Loop • Blood glucose concentrations rise after a sugary meal (the stimulus), the hormone insulin is released and it speeds up the transport of glucose out of the blood and into selected tissues (the response), so blood glucose concentrations decrease (thus decreasing the original stimulus).

11. Homeostasis of Blood Pressure • Baroreceptors in walls of blood vessels detect an increase in BP • Brain receives input and signals blood vessels and heart • Blood vessels dilate, HR decreases • BP decreases

12. Positive Feedback during Childbirth • Stretch receptors in walls of uterus send signals to the brain • Brain induces release of hormone (oxytocin) into bloodstream • Uterine smooth muscle contracts more forcefully • More stretch, more hormone, more contraction etc. • Cycle ends with birth of the baby & decrease in stretch